The present invention relates to a method and apparatus for treating spent liquor from the pulp industry, whereby the spent liquor is introduced into a mixing tank for adding ash and/or other substances to the spent liquor.
The spent liquor, i.e. black liquor produced in the manufacture of chemical pulp in the pulp industry, is normally incinerated in a chemicals and heat recovering boiler. In a conventional recovery boiler, the process chemical is recovered by injecting liquor into the furnace, whereby the liquor dries rapidly and burns in reducing conditions, producing a melt. The melt primarily contains sodium carbonate, sodium sulfide and sodium sulfate, however a large number of other compounds are also present. The main components, Na.sub.2 CO.sub.3 and Na.sub.2 S, are essential to the manufacture of cellulose pulp, particularly kraft pulp.
The flue gases from the recovery boiler contain abundant quantities of ash ("fly ash"), which is typically separated in an ash hopper and electrostatic filters prior to exhausting the flue gas from the boiler.
The main component of this fly ash is sodium sulfate. Usually, the fly ash is recycled to the soda recovery boiler by mixing the ash with the strong black liquor which is to be introduced into the boiler. However a significant problem area in the black liquor treatment process is the mixing of fly ash and make-up chemical (e.g. Na.sub.2 SO.sub.4), if any, with the black liquor. Conventionally, a separate, fairly large mixing tank is provided, in which the ash is mixed with the black liquor coming from evaporators of an evaporation plant (via a storage tank) into the mixing tank. Black liquor is also returned from the liquor circulation of the boiler to the mixing tank.
The black liquor storage and treating system of the prior art has many problems associated therewith. When returned from the boiler to the mixing tank, the hot black liquor emits vapors which contain substantial quantities of impurities. Vapor is carried to the ash transportation means where it causes malfunctions, requiring frequent maintenance and cleaning of the system. The costs are high because of the separate, large mixing tank. There are also a large number of pumps and much piping. As the whole system is complicated and requires numerous pieces of equipment, operating and maintenance costs are high.
According to the present invention, a method and apparatus for storing and treating of black liquor which are simpler, more economical, and more reliable in operation than the prior art described above, is provided. A feature of the method according to the present invention is that the mixing tank is connected to the spent liquor storage tank so that mixed liquid is discharged directly into the storage tank. A feature of the apparatus according to the invention is that a volume is provided with a mixer connected to the spent liquor storage tank, the storage tank defining a part of the mixing volume.
The invention essentially improves the reliability in operation of the mixing procedure. The ash and salt cake mix more uniformly with the black liquor, vapor emissions do not cause the types of problems of conventional mixing tank systems, and the system is simple and reliable. Several pumps and pipes necessary in the prior art system may be eliminated.
One aspect of the invention comprises: A spent liquor storage tank having a liquid outlet from a bottom portion thereof, and including a sidewall defining an interior. Means defining a mixing tank having a much smaller interior volume than the storage tank, the means including the sidewall of the storage tank. Means for mixing fly ash and spent liquor in the mixing tank. Means for introducing fly ash and spent liquor into the mixing tank. And, means for discharging mixed fly ash and spent liquor from the mixing tank directly into the interior of the storage tank.
Another aspect of the invention comprises: A black liquor storage tank having a top and a bottom, including a liquid outlet adjacent the bottom thereof, and an interior volume. A recovery boiler connected to the liquid outlet, and producing fly ash. A recirculating conduit connected from the recovery boiler to the storage tank interior volume. A mixing tank remote from the recirculating conduit having a discharge opening in direct communication with the storage tank interior volume. Black liquor evaporators. Means for connecting the black liquor evaporators to the mixing tank for introducing black liquor from the evaporators into the mixing tank. Means for introducing fly ash from the recovery boiler into the mixing tank. And, mixing means associated with the mixing tank for mixing black liquor and fly ash therein to produce a liquid mixture that is discharged through the discharge opening directly into the storage tank interior volume.
Another aspect of the invention comprises the following method steps for treating spent liquor from pulping: (a) Concentrating the spent liquor. (b) Feeding the concentrated liquor into the mixing volume. (c) Introducing fly ash, and optionally makeup chemical, into the mixing volume. (d) Mixing the concentrated liquor, fly ash, and optionally makeup chemical, in the mixing volume to provide a liquid mixture. (e) Discharging the liquid mixture directly into the interior of the storage tank from the mixing volume, without utilizing any conduits external of the storage tank. (f) Withdrawing liquid mixture from the storage tank and introducing it into the recovery boiler to be burned. And, (g) recirculating a part of the liquid mixture from the recovery boiler back to the storage tank. The invention provides a simplified and advantageous method and apparatus for handling black liquor or like spent liquor.